Technical Abstract:
The genus Sorghum has 25 species forming two lineages: one with 2n=10 relatively large genome species and polyploid relatives (subgenera Para-sorghum and Stiposorghum); the other with 2n=20 and 2n=40 species with smaller genomes (subgenera Eu-sorghum, Chaetosorghum, Heterosorghum). Strong reproductive isolation barriers have prevented successful hybridization of S. bicolor with any Sorghum species classified in the subgenera other than Eu-sorghum. Pollen-pistil incompatibility is the primary reason why hybrids can not be produced. There is a reduction of alien pollen germination and tube growth in sorghum pistils. This incompatibility mechanism was overcome by using a S. bicolor genotype that allows growth of pollen tubes of alien Sorghum species. Hybrids have been recovered after dusting the stigmas of S. bicolor with pollen from diverse species such as S. angustum, S. nitidum, and S. macrospermum. Analysis of chromosome pairing in the S. bicolor x S. macrospermum hybrid showed homology between the two species. This, along with additional cytogenetic, morphological, and DNA sequence comparisons support the hypothesis that S. bicolor is phylogenetically closer to S. macrospermum than it is to species of the subgenera Para-sorghum and Stiposorghum.